MFL signals are generated due to geometric changes and magnetic changes. For example, part of the signal generated at a mechanical damage defect is due to geometric changes - a reduction in wall thickness due to metal loss causes flux to leak out. Some of the signal is due to the separation of the sensor from the pipe (lift-off), which can be minimized by a good sensor carrier system. The rest of the signal is largely due to magnetic changes - changes in magnetization properties that result from stresses, strains, or damage to the microstructure of the steel.

MFL signals at mechanical damage

The plot on the left is a typical MFL signal from metal loss. Flux, which is normally carried by the pipe wall, "leaks" in regions where the wall thickness is reduced. The sensor records an increase in flux level at the reduced-thickness area. Metal loss signals have a characteristic increase in measured field along the defect, with a slight decrease at both ends. For very long defects, there can be a dip in the signal in the center part of the defect.

The plot in the center is a typical MFL signal from a dent. Here, the signal shape is fundamentally different than that seen at metal loss. The signal is due to two effects that occur at the same time. First, the sensor orientation relative to the local pipe wall changes. The sensor still records the axial field but the pipe wall is no longer parallel to the sensor; since the flux field is a vector quantity, the resultant measurement changes. Second, residual stresses and strains change the local magnetic properties. Dent signals show characteristic peaks near the start and finish of the dent with a relatively low signal through the defect.

The plot on the right is a typical MFL signal from a cold worked region. Here, the signal shape is fundamentally different from that of both metal loss or a dent. Flux in the region immediately below the cold worked area decreases. This change occurs because the cold worked region, which is on the side opposite the sensor, carries more flux, thereby reducing the flux in the rest of the pipe. In addition, there is a slight increase in signal at either end. These two signal features are characteristic of mechanical damage.